1. Field of the Invention
The invention relates to material for a chemical vapor deposition (CVD) method for forming an oxide-system dielectric thin film used for a dielectric memory and a dielectric filter and the like, and also relates to a capacitor for a memory device formed by the material.
2. Description of the Prior Art
In recent years, the integration of semi-conductor memory devices has rapidly advanced. For example, regarding the dynamic random access memory (DRAM), bit number has increased at the sudden pace of, for example, 4 times in 3 years.
This is because of attaining a high speed of device, and low consumption power and low cost. But, even if an integration degree is improved, a capacitor which comprises one of the elements in the DRAM must have a certain value of capacitance. From this reason, it is necessary to cause the film thickness of capacitor material to be thin. Therefore, the thickness of the thin film using SiO2 which has been used so far is limited for thinner film. If the dielectric constant increases by changing material, it is able to obtain thinner film as well as a certain amount of capacity.
Accordingly, studies for the dielectric material which has a large dielectric constant and is used as a capacitor for a memory device become the object of public attention these days.
Regarding the performance required for capacitor material, it has been most important that the material is a thin film which has a large dielectric constant and has a small leak current. That is, it is necessary to use a large dielectric constant material and also a thinner film and to make the leak current be minimum. For a general development aim, it is desirable that the SiO.sub.2 equivalent thickness of the thin film be less than 1 nm and the leak current density be less than on the order of 10.sup.-8 A/cm.sup.2 when 1.65 V is applied to the thin film. For forming a thin film on the capacitor electrode of the DRAM having stage difference, it is profitable for the process to cause the thin film to stick well around the complex shape of the body using the CVD method. From the above points of view, an oxide-system dielectric thin film such as tantalum-oxide, lead-titanium zirconate (PTZ), lead-lanthanum-zirconium titanate (PLZT), strontium titanate and barium titanate are examined for forming a thin film using all kinds of film formation methods. Though it is most advantageous to form a film using the CVD method, it is troublesome that there is no material as a raw material compound (referred to raw material, below) having good stability and vaporization characteristics for CVD. This is because the heating vaporization characteristics of the dipivaloyl-methane (DPM) compound of .beta.-diketonate-system is principally not good, which is used a lot as a raw material for CVD. It is pointed out that this is a substantial defect caused by essential instability of the metal DPM compound, for example, in the 52th Applied Physics Association Seminar preliminary report No. 9a-P-11. As shown in the report No. 9a-P-11, the CVD method is still studied in spite of the above defect. It happens at an extreme occasion that the raw material has to be thrown away during the film forming when the raw material is unstable. Accordingly, when the raw material mentioned above has the defect, good performance of the dielectric thin film and good manufacturing repeatability are not obtained at present.
As described above, regarding the method for manufacturing an oxide-system dielectric thin film by conventional CVD method, since the CVD material has less stability and bad vaporization, it is impossible to heat the CVD raw material at a low temperature and transport it stably to the CVD reactor.
Therefore, it is difficult to control the composition and also there is a big problem that a dielectric film can not be formed stably in order to get good characteristics. On the other hand, if the raw material is heated at a high temperature in order to increase the vaporization efficiency, thermal decomposition occurs during transporting the raw material. Therefore, a crystallization failure of the film or composition drift inevitably occurs. In addition, the raw material is thrown away inconveniently as described above. If the composition (reaction) time becomes long by suppressing vaporization speed, the vaporization state of the raw material changes as time goes by. Therefore, the film composition becomes heterogeneous toward the thickness direction, and thus the leak current inevitably increases. For this reason, the development is strongly expected to obtain CVD material having good and stable vaporization in a low temperature heating if the CVD material is used for a number of times and a long period. But, there is still no further progress with respect to the technique.
Therefore, the CVD material for forming an oxide-system dielectric thin film of the present invention is invented to eliminate the above defect which occurs in raw material used in the conventional CVD. According to the material of the present invention, it is different from the conventional material in that the conventional multi-source materials need to be separately vaporized, while in the present invention the material are liquidized and at the same time vaporized and transported stably to the reactor. Accordingly, it is able to form the dielectric thin film for a capacitor having a good performance with good repeatability. DPM compound which is used frequently in the conventional CVD method as described above, it is proved that these compounds of CVD raw material for oxide-system dielectric thin film capacitor of the present invention, especially, such as the alkaline earth metal of Ba and Sr and the solid state compound of Pb and Ti, is not in a stable state and also not in a good vaporization state. Therefore, when the oxide-system dielectric thin film comprised of these metal oxide is formed by a CVD method, it is proved for a multi-source material that it is difficult to control an object composition and to form a film continuously.
Accordingly, in the present invention, the inventors found that the controllability of the composition is improved and a dielectric film having desired characteristic is formed continuously with good repeatability by dissolving these compounds in the solvent which includes tetra-hydrofuran as a main component for maintaining these compound stable for a long period in order to make one solution, and by evaporating the solution in a relatively low temperature heating without heat decomposition. Especially, in this invention, the repeatability for forming the film is remarkably improved in case that the film is formed continuously if liquid raw material is used when forming an oxide-system dielectric thin film in the multi-component materials. Therefore, the performance of the dielectric film is remarkably improved by using the dielectric film as a capacitor used for memory devises.
Regarding many kinds of organic solvents other than the tetra-hydrofuran, the inventors investigated in detail in relation to the dissolving capacity of the material for composing the dielectric thin film comprised of the solid organic metal (organometallic) compound, a evaporation characteristic of the solution and a long life stability. The result shows that there found many solvents which could dissolve the solid material well. But there found no good solvent having good heat vaporization and long life stability when making a solution by dissolving the raw material compound used as the dielectric film comprised of the organic metal compound such as the tetra-hydrofuran.
Therefore, the CVD raw material of the present invention is used for forming the oxide-system dielectric thin film having single or multi-components, and which is liquid raw material in which at least one kind of organic material is dissolved in the tetra-hydrofuran and also can be attained in a stable state and in a good repeatability of evaporation by heating. Especially, in the present invention, the same raw material can be used for a number of times without reducing the evaporation characteristic, which was impossible to be carried out so far.
The above explanation is also applied to liquid material in which the organic metal compound is dissolved in the solvent including the tetra-hydrofuran.
It is desirable to use at least one kind of metals selected among Pb, Ti, Zr and alkaline earth metal as the metal atom of the organic metal compound.
It is desirable to use a compound wherein the metal atom is coupled with organic groups through oxygen atoms as the organic metal compound.
Regarding the compound wherein the metal atom is coupled with organic groups through oxygen atoms, it is desirable to use at least one kind of metals selected among Pb, Ti, Zr and alkaline earth metal as the metal atom.
Regarding the compound wherein the metal atom is coupled with organic groups through oxygen atoms, it is desirable that the metal atom comprises Sr and/or Ba and Ti.
Regarding the compound wherein the metal atom is coupled with organic groups through oxygen atoms, it is desirable to use acetyl-acetonates, dipivaloyl-methanates, alkoxides, hexafluoro-acetyl-acetonates, penta-fluoro-propanoil-pivaloyl-methanates, cyclopentadienyl which are selected at least among groups of Pb, Ti, Zr and alkaline earth metal, or one or more kinds of those derivatives.
Regarding the compound wherein the metal atom is coupled with organic groups through oxygen atoms, it is desirable to use Sr and/or Ba and Ti dipivaloyl-methanate system compound.
Regarding the compound wherein the metal atom is coupled with organic groups through oxygen atoms, it is desirable to use Sr and/or Ba dipivaloyl-methanate system compound and Ti alkoxide.
Regarding the compound wherein the metal atom is coupled with organic groups through oxygen atoms, it is desirable to use Sr and/or Ba dipivaloyl-methanate system compound and Ti iso-propoxide.
In the capacitor used for memory devises of the present invention, the dielectric film for the capacitor is formed by CVD method using tetra-hydrofuran or CVD raw material for oxide-system dielectric thin film in which the organic metal compound is dissolved in the solvent including the tetra-hydrofuran.